Filter out free regions too small for alignment of partitions.

Make sure that we don't even look at regions shorter than the alignment
grain size, and that free regions have sufficient length after aligning
the start and end sector before instantiating a DiskChunk.

(cherry picked from commit 4174f20f2aa6f66745c736a83f5fb09a96b4e2f0)

Related: rhbz#1083687